Astronomers unveil secrets of giant elliptical galaxies

12 September 2014 Davor Krajnović, astronomer at the Leibniz Institute for Astrophysics Potsdam (AIP), and his colleagues Eric Emsellem (ESO) and Marc Sarzi (University of Hertfordshire), have discovered how giant elliptical galaxies move. Their study is based on data obtained by the newly installed Multi Unit Spectroscopic Explorer (MUSE).

Velocity map of M87.

The three astronomers observed the giant galaxy M87 (NGC4486), which is the central galaxy in the Virgo cluster, and discovered that it displays some bulk rotation, albeit of a very low amplitude. The precision of MUSE allowed the team to reveal that the stars of M87 can move around its centre with average velocities of just 10-20 km/s. Equivalent to 36-72,000 km/h, this speed may seem very high, but for galaxies this is extremely slow.

Elliptical galaxies have long been considered as essentially being made up of old stars that move randomly within them, like a swarm of bees. This has been challenged in many instances in the past ten-twenty years, but giant elliptical galaxies are still considered as a nearly round and non-rotating group of old stars.

By showing that a "simple" galaxy like M87 can be quite complicated in the eyes of the new MUSE spectrograph, this result demonstrates the potential of this new instrument for further advancing our understanding of galaxies, and their formation. Davor Krajnović states: “MUSE has the capability to enhance our understanding of galaxies, how they form and develop. By using the MUSE velocity data to constrain simulation models, we might reach a whole new level of precision.” Their work is published in the Monthly Notices of the Royal Astronomical Society and a pre-publication version of the paper is available on arXiv: http://arxiv.org/abs/1408.6844.

The Multi Unit Spectroscopic Explorer (MUSE) is a 3D-spectrograph for the Very Large Telescope (VLT) of the European Southern Observatory at Paranal (Chile). MUSE features a complex optical system with the capacity to split and slice a field that measures one square arcminute on the sky into 90,000 spatial elements. For each point a spectrum is created, covering the optical and near infrared wavelength region of 465-930nm. AIP provides the Data Reduction Software and operates one of the data centres accessible to scientists from all over the world.

(Click to enlarge)

Left: Image of M87: Some small companions galaxies of this giant and round elliptical galaxy are visible to the right of the image. The red square delineates the field-of-view of the MUSE instrument, where the velocity of the central stars of M87 have been measured.

Right: A map for the average velocity of the stars in the central region of M87, divided in polygonal regions where the MUSE data have been combined to reach a sufficient quality for these measurements. After accounting that M87 as a whole is moving away from us, red or yellow bins show stars that on average are receding whereas blue or light azure bins show stars that on average are approaching. The MUSE map reveal a complex motion of the stars in M87, where stars move in one way in the central region and in another in its outskirts.

The key topics of the Leibniz Institute for Astrophysics Potsdam (AIP) are cosmic magnetic fields and extragalactic astrophysics. A considerable part of the institute's efforts aim at the development of research technology in the fields of spectroscopy, robotic telescopes, and e-science. The AIP is the successor of the Berlin Observatory founded in 1700 and of the Astrophysical Observatory of Potsdam founded in 1874. The latter was the world's first observatory to emphasize explicitly the research area of astrophysics. Since 1992 the AIP is a member of the Leibniz Association.